1. Field of the Invention
The present invention relates to a method for coating a polymeric substrate, and more particularly, relates to a method for applying a hydrophilic coating to a polymeric substrate to provide a lubricious surface to the substrate, and to articles having a lubricious surface prepared thereby.
2. Description of the Prior Art
Materials intended for fabrication of biomedical articles such as heart valves, catheters, intrauterine devices, wound drains and the like require a high degree of compatibility with body fluids and tissues. Some compatibility is afforded by hydrogels. These gels are polymeric in nature and swell with water to provide hydrophilic surfaces which have found wide application for various biomedical devices, particularly contact lenses. For some applications, however, it would be highly desirable to provide a surface having, in addition to hydrophilicity, a low coefficient of friction when in contact with an aqueous-based fluid, such as a body fluid. Such a low-friction surface would aid insertion or removal of a device into or out of a patient, and would contribute to patient comfort. Further, for ease of handling by a technician, it would be desirable if the surface retained a normal feel when dry and demonstrated its low-friction properties only when in contact with the aqueous-based medium.
Several methods have been used for attainment of low-friction surfaces on polymeric substrates. U.S. Pat. No. 4,381,008 to Thomas et al. discloses increased lubricity resulting from application of extrusion or stretching procedures to polymeric substrates. Coatings of various materials, such as silicone, fluorocarbon, or cellulose have been applied to polymeric substrates to decrease surface friction. Exemplary of such methods is that disclosed in U.S. Pat. No. 4,169,163 to Judd et al.
Polyvinyl pyrrolidone (PVP) is a water soluble hygroscopic polymer having excellent compatibility with body tissues. It is often copolymerized with another material to modify the PVP properties. U.S. Pat. No. 3,216,983 to Shelanski et al. discloses copolymers of PVP and various polyisocyanates which retain many of the desirable properties of PVP while reducing the water solubility and increasing adhesion of the copolymer to wood, glass, metal and the like. U.S. Pat. No. 3,939,049 to Ratner et al. discloses the grafting of PVP to polymer substrates after prior activation of the substrate surface by ionizing radiation. There is no teaching in the Ratner disclosure of surface lubricity.
U.S. Pat. No. 4,119,094 to Micklus et al. discloses a method to place a polyurethane - PVP interpolymer on a polymeric substrate and thereby provide a low friction surface to that substrate. The method of the Micklus et al. patent requires initial application of a polyisocyanate to the substrate, and the substrate must be one to which conventional polyurethanes adhere.
U.S. Pat. No. 4,373,009 to Winn discloses a modification of the Micklus et al. procedure in which the substrate is coated with a hydrophilic copolymer of PVP and a polymer containing active hydrogens. The substrate may be any material to which cured isocyanates adhere. The active hydrogens react with the polyisocyanate to form covalent bonds which are said to provide a more durable coating on the substrate than obtained by previous methods.
The surfaces prepared in accordance with the prior art methods described above do provide surface lubricity to the articles or substrates described, but all of these prior art methods still have disadvantages. In some, the surfaces are insufficiently lubricious for use as biomedical articles. Others require additional steps to introduce the desired surface lubricity, such as activation of the surface by radiation or use of an intermediate polyisocyanate bonding material between the substrate and the coating. Such additional steps increase the complexity and cost of manufacturing operations. Further, a PVP coating bonded to a polymeric substrate with a polyisocyanate, when in contact with water, comes off the substrate surface in flakes so that the aqueous medium becomes cloudy. Thus, there remains a distinct need for a better method to provide polymeric substrates with lubricious surfaces.